How to change horizontal category on a data series or manually create a legend

Although I can not think of a way to produce the chart exactly as you want, perhaps the attached file will give you or one of the other contributors an idea about how to get what you want.
1. Produce a pivot table with Experiment and Inkubation placed in the Rows area and Differens (Average of) in the Values field
2. Produce a pivot chart from the pivot table.
Let us know if you have any questions.